Baking two objects to one texture
Ok, so I assume that one leaf was made, used as a particle system which was applied as an object and then this leaves object was already joined to ground object to create one object but the vertices of the leaves have the leaf texture and the ground vertices have the ground texture. Is that correct?

It looks like you already have each thing unwrapped into one UV map so if I've understood you correctly baking two assigned materials into one diffuse material is what you want to do, correct?

Baking two material textures into one: 

1) Open the node editor and the UV image editor. In the UV image editor create a new image and give it the dimensions you want your new diffuse image texture to be. Make sure the dimensions are quite big because it gives you the option to scale it down if you want to crop it. Now save it to a location you can remember.

2) Go to node editor, click on your object and go into material of either object and add an image texture. DO NOT CONNECT IT TO ANYTHING. Now open your new blank image inside the image texture you added (the image which you created and saved earlier).

3) In the render tab, scroll down until you see "Bake". Expand the tab and choose Diffuse (it should be the default option). With the image texture selected in the node editor (make sure it's highlighted) click "Bake". This should create a new material which includes the leaf materials for each leaf plane in your object.

4) Now you should be able to delete the leaf planes and use the new texture on your ground plane. If you want to, use GIMP or photoshop to crop the newly baked image so that there are no transparent parts. Now you can UV unwrap the ground plane again so it fills the UV map and align it to the newly cropped baked image texture so it all lines up if it doesn't already.

You can also create a normal map using the same method by duplicating the ground plane, deleting the leaf geometry in edit mode and UV unwrap it. In the bake tab, choose "Normal", select your complete mesh and shift select your new edited ground plane mesh second and in the bake tab click the option "Selected to Active". Then click "Bake".

Remember to duplicate everything and move it to a different layer or save a back up file before you do anything so that you don't lose everything if you make a mistake.

If I've missed a step, please feel free to edit my answer because I'm away from my PC and can't double check :)

Hope this is what you were looking for :)